MOSFET (metal oxide semiconductor field effect transistor) devices are often fabricated having three separate terminals, with those terminals being the source, the gate, and the drain. In these devices, the source and body regions are typically shorted to one another.
In other designs, however, the MOSFET device is fabricated having four separate terminals, with the fourth terminal being the body terminal. A typical four-terminal MOSFET structure is shown in FIG. 1. This structure contains a P-type body region 102 with P+ body contact region 103, an N+ source region 104, an N+ drain region 106 and a gate region, which consists of a doped polycrystalline silicon conductive region 108 and a gate dielectric layer 109. An insulating layer 110 is provided over the conductive region 108.
In many applications, it is important to control the voltage of the body with respect to the source under all conditions. For example, as is well known in the art, there is a parasitic bipolar transistor that is intrinsic to the MOSFET device. Referring to FIG. 1, for example, the parasitic bipolar NPN transistor is intrinsically formed from the N-type source region 104 as the emitter, the P-type body region 102 as the base and the N-type drain region 106 as the collector. This parasitic NPN transistor can become active when the source-to-body voltage within the MOSFET exceeds the forward voltage of the PN junction that exists between the source and body regions. Activation of the parasitic transistor can lead to latchback, interfering with the intended operation of the MOSFET as well as the circuit that contains the MOSFET.
By supplying separate body contacts within the device, the voltage of the body with respect to the source can be controlled. Alternatively, the source and drain terminals of the device can be allowed to exchange their functions, thus permitting current to flow in one direction at some times, and a second direction at other times. In either case, the adverse effect of the parasitic transistor within the device is dealt with.
In a MOSFET device that is intended to supply large currents or to switch rapidly, the number of body contacts and their locations can be critical to the prevention of latchback, and hence to the successful operation of the device. Various geometries have been proposed for MOSFET devices having separate body contacts. For example, cellular geometries with body contact regions located at the boundaries of the MOSFET array, as well as between regions of the array, have been proposed. Interdigitated source and drain regions with separate body contact regions are another example.
The present invention is directed to a particularly effective family of MOSFET device designs in which body contact regions are brought into close proximity with the source and drain regions.
According to an embodiment of the invention, a MOSFET device is provided that comprises: (a) a body region; (b) a plurality of body contact regions; (c) a plurality of source regions; (d) a plurality of drain regions; and (d) a gate region, wherein, in plan view, the source regions and the drain regions are arranged in orthogonal rows and columns, and at least a portion of the body contact regions are bordered by four of the source and drain regions, preferably two source regions and two drain regions.
In more preferred embodiments, the MOSFET device comprises: (1) a semiconductor region of first conductivity type having an upper surface; (2) a plurality of source regions of a second conductivity type formed within an upper portion of the semiconductor region adjacent the upper surface; (3) a plurality of drain regions of the second conductivity type formed within an upper portion of the semiconductor region adjacent the upper surface; (4) a plurality of body contact regions of the first conductivity type formed within an upper portion of the semiconductor region adjacent the upper surface, the body contact regions having a net doping concentration that is higher than that of the semiconductor region; and (5) a gate region disposed over the upper surface of the semiconductor region, the gate region comprising (a) a gate electrode region and (b) a gate dielectric layer disposed between the gate electrode region and the semiconductor region. When viewed from above the upper surface (i.e., in plan view), the source regions and the drain regions of this MOSFET device are arranged in orthogonal rows and columns, and at least a portion of the body contact regions are bordered by four of the source and drain regions, more preferably two source regions and two drain regions.
Preferably, the semiconductor region is a silicon semiconductor region, the first conductivity type is P-type conductivity, and the second conductivity type is N-type conductivity. The gate electrode preferably is a doped polysilicon electrode, and the gate dielectric preferably is silicon dioxide.
In preferred embodiments, the source regions and the drain regions are provided in an alternating arrangement within the orthogonal rows and columns.
In some embodiments, when viewed from above the upper surface, the source regions and the drain regions are in the shape of octagons. The octagons can be, for example, regular octagons or elongated octagons having two planes of symmetry.
Similarly, when viewed from above the upper surface, the body contact regions are in the shape of octagons in some embodiments. In other embodiments, the body contact regions can be in the shape of squares or diamonds when viewed from above the upper surface.
The ratio of source regions to body contact regions can vary. For example, each source region can be provided, on average, with (a) one adjacent body contact region, (b) two adjacent body contact regions or (c) four adjacent body contact regions.
In preferred embodiments, a multilayer interconnect structure is provided over the MOSFET device.
One advantage of the present invention is that a MOSFET device design is provided, which effectively addresses the problems arising from the parasitic bipolar transistor that is intrinsic to the device.
Another advantage of the present invention is that body contact regions can be provided throughout the MOSFET device, with little loss of shared source/drain perimeter area, and hence with little loss in current density.
These and other embodiments and advantages of the present invention will become immediately apparent to those of ordinary skill in the art upon review of the Detailed Description and Claims to follow.